In heating boilers, which are referred to as calorific value boilers, the flue gases from the furnace are cooled below the dew point, so that the water vapor present in the flue gases condenses and precipitates together with the pollutants and other combustion residues present in the flue gases. Because of the sulfur present in these liquid or gaseous fossil fuels, this condenstate contains sulfur oxides and is, therefore, highly acidic.
In the known calorific value boilers for residential heating systems, this condensate is neutralized by raising the pH value and discharged into the sewer. The oil droplets reaching the boiler especially when the burner is turned on and off and the dissolved and undissolved hydrocarbons present in the condensate as a result of incomplete combustion also wind up in the sewer and contaminate the environment. To clean a boiler, it is standard practice after a dry cleaning to send the residues obtained, which are high in pollutants, to the dump, where they lead to environmental pollution; whereas, when a wet cleaning method is sued, the pollutants wind up for the most part in the sewer system and hinder the biological clarification of the waste water.